An understanding of the common approach taken to microstructural fabrication can be obtained by reference to the above cited co-pending patent application Ser. No. 08/787,281. In that application, a basic silicon "micromachining" technique is described as a means for accurately fabricating small structures or devices. In particular, such processing involves a selective surface processing of a silicon substrate to form an anchored cantilever microstructure, as well as the selective deposition thereon and patterning of thin film layers of dielectrics, semiconductors and similar materials. Electrothermal control of the movement of the cantilever is disclosed.
It will be appreciated that the microstructures described in the referenced application 08/787,281 exhibit substantial ranges of motion of the free ends of the cantilevers involved such that latched switching action can be readily obtained.
A particular field of application for microstructures has been that in which the cantilever device functions to record whether or not a certain acceleration was applied to a given mechanism--without requiring battery voltage. This is especially advantageous for use in monitoring parcels, boxes, and crates in shipment and for care in handling and the like. See, in particular, U.S. Pat. No. 5,891,255 to Ciarlo in which there is disclosed a bulk machining method for etching a &lt;110&gt; silicon wafer to produce first and second, orthogonally oriented, cantilever beams, which bend parallel to the surface of the wafer in response to acceleration events. Ciarlo uses a bulk micromachining action wherein the preferential etch rate of a silicon wafer is used to enable an etch action to go completely through a wafer so as to create the oriented beam structure. The length of the first cantilever structure forms an angle with the length of the second cantilever, where the value of the angle is substantially 70.5.degree. or 109.5.degree. and no other orientations are permissible. The apparatus disclosed therein provides a specific configuration and a relationship between the first and second silicon semiconductor cantilever beams in fulfilling the objective of obtaining a latched state for the device or apparatus.
Surface micromachining is widely used in the configuration of semiconductor devices. Such surface micromachining comprises the steps of depositing a material layer, applying and patterning a photoresist layer and using the patterning to enable selective etching or deposition of dopants. A repetition of these steps enables the creation of a three dimensional semiconductor arrangement which is inexpensive and easily controllable.
Accordingly, it is a primary object of the present invention to provide single or multiple latching effects in a microaccelerometer, effectuating thereby the recording of changing acceleration values.
Another object is to fulfill the primary object in ways that are compatible with standard surface micromachining processes.
Another object is to permit sensing of acceleration vectors over a range of directions beyond limitations of the prior art.
An ancillary object is to record the acceleration event by means of a variety of "readout" devices or schemes in which the mechanically latched state is realized and made apparent to an observer directly, or by means of electrical or optical techniques
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawings, wherein like parts have been given like numbers.